The process of producing pure alumina from bauxite (the Bayer process) has not significantly changed in the last 100 years. The Bayer process can be considered in three stages: (1) leaching, (2) precipitation and (3) calcination.
In the leaching stage, aluminum-bearing minerals in bauxite (e.g., gibbsite, böhmite and diaspore) are selectively leached from insoluble components by dissolving them in an aqueous solution of sodium hydroxide:(gibbsite): Al(OH)3+Na++OH−→Al(OH)4−+Na+(böhmite and diaspore): AlO(OH)+Na++OH−+H2O→Al(OH)4−+Na+Before being subjected to the Bayer process, the bauxite ore is pulverized and milled to reduce the particle size, which increases the available surface area for contact with the sodium hydroxide. The crushed bauxite is then combined with the process liquor and delivered as a slurry to a heated pressure digester vessel. Conditions within the digester vessel (e.g., concentration, temperature and pressure) are set according to the properties of the bauxite ore. Ores with a high gibbsite content can typically be processed at about 140° C., while processing of ores with a high böhmite content typically requires temperatures between about 200° C. and about 240° C. The pressure is defined by the steam pressure during actual process conditions. At 240° C., the pressure is about 35 atmospheres.
After the leaching stage, the insoluble bauxite residue must be separated from the dissolved alumina-containing liquor by a combined process of precipitation and settling. The liquor is typically purified through a series of filters before being transferred to the precipitators.
The alumina in the alumina-containing liquor is precipitated by cooling in the form of aluminum trihydroxide (gibbsite):Al(OH)4−+Na+→Al(OH)3+Na+OH−The gibbsite precipitation step is basically the reverse of the leaching step, except that the nature of the aqueous media alumina product is carefully controlled by reaction conditions, including seeding or selective nucleation, precipitation temperature and cooling rate. During the precipitation process sodium hydroxide is regenerated for additional alumina leaching. The purified crystalline gibbsite, which is also referred to as a hydrate, is then separated from the liquor and calcined to form alumina for the aluminum smelting process.2Al(OH)3→Al2O3+H2O
As the result of Cold War weapons material production, large volumes of radioactive and chemically hazardous aqueous wastes, which include aluminum oxides, have been generated at the U.S. Department of Energy (DOE) facilities. These wastes are stored in storage tanks at various locations, for example the DOE Hanford site in Washington State. At present, the DOE Hanford site stores approximately 53 million gallons of radioactive aqueous waste in approximately 177 underground tanks. This waste must be processed in the Hanford Waste Tank Treatment and Immobilization Plant (WTP) to immobilize (vitrify) the radioactive waste constituents.
One processing problem at the Hanford site relates to the presence of aluminum oxides (e.g., alumina) in the aqueous waste that forms a sludge that impairs transfer and processing of the waste. Over 44,00 metric tons of sodium salts are present in the aqueous waste and an additional estimated 30,000 metric tons of sodium as sodium hydroxide are required to leach (i.e., solubilize) insoluble alumina sludge. This additional amount of sodium salts increases both the glass volume and treatment schedule at the WTP. Further, soluble alumina can plug WTP process equipment by forming amorphous gels during any of a number of processes (e.g., filtration, ion-exchange, cooling, dilution and/or neutralization) utilized in WTP operations.
The inventors have discovered that the addition of lithium salts and/or magnesium salts to aqueous media containing aluminum oxides results in the formation of low solubility lithium-aluminate complexes and/or magnesium-aluminate complexes that provide a quick and effective way of separating soluble alumina from the aqueous media. This novel process is well-suited to address the aqueous waste problems present at the Hanford site. The separated lithium-aluminate complexes and magnesium-aluminate complexes may be beneficially used as glass formers in the production of vitrified low activity waste (LAW).
The advantages of using lithium salts and/or magnesium salts to remove aluminum oxides from aqueous media, are at the very least as follows: (1) the per-pass yield for the precipitated lithium-aluminate complex or the magnesium-aluminate complex is greater than for a modified Bayer process; (2) removal of aluminum oxides by lithium and/or magnesium as described herein does not require seeding or seed recycle; and (3) the precipitation of lithium-aluminate complexes and magnesium-aluminate complexes forms large crystals that may be readily beneficially separated and decontaminated from aqueous media.